It has become common place in the technology of plant husbandry and plant research to cultivate plant tissues in vitro for a wide variety of purposes. The purposes include conducting experiments with plant tissues, genetically engineering plant tissues, the asexual reproduction of commercial flowering and decorative plants, and other similar purposes. It is generally known that certain nutrient components such as salts, vitamins and hormones are necessary to foster plant tissue growth in or on a culture medium and various different mechanisms are used to provide plant tissues grown in culture with such requirements.
Currently, the vast majority of plant tissue culture methods employ a solid phase medium as a growth substrate for plant calli and plant tissue explants. The typical solid or semi-solid medium utilized is an agar type substrate, such as that described in Gamborg and Shyluk, "Nutrition, Media and Characteristics of Plant Cell and Tissue Cultures", Plant Tissue Culture: Methods and Application in Agriculture, Academic Press, 1981; Helgeson, "Plant Tissue and Cell Suspension Culture", Tissue Culture Methods for Plant Pathologists, Blackwell Scientific Publishing, 1980, page 19. Using an agar substrate, the plant tissues themselves are placed directly on the solid medium which is dosed with the nutrient requirements of the plant cells growing thereon. It is recognized, however, that many such nutrients and also cellular metabolites do not diffuse as effectively across a solid or semi-solid agar matrix as compared with their rates of diffusion in aqueous solution. Therefore, around plant cell cultures grown on a solid phase agar substrates, chemical gradients of both nutrient components and cellular metabolites tend to be generated in the agar layer surrounding the cultured plant tissues. These gradients can cause undesirable or imbalanced chemical conditions in the environment immediately about the growing plant callus or tissue which can sometimes interfere with effective continuous growth of the cultured plant tissues. The deleterious effects of the development of these gradients can be partially reduced by frequent transfer of the callus or culture tissues to fresh culture plates. This practice is, however, relatively expensive and laborious since the tissue cultures must be individually transferred by hand from new to old plates. Furthermore, the continuous growth of the cultured specimens is often disturbed, sometimes irreparably, because of this manipulation, and the handling of the cultures increases the chance of microbial contamination of the growing cultures and their media.
One other previously used method of fostering the growth of plant cell cultures or calluses is to grow the cultures on filter paper bridges folded and inserted into a culture vessel, such as a test tube, supplied with a quantity of liquid nutrient medium. The liquid medium travels up the filter paper bridge by capillary action and thus provides nutrients to the plant tissues or callus cultures grown on the filter paper bridge. This method does provide a better exchange of nutrients between the culture and the medium than does the use of solid or semi-solid supports like agar and agarose. This method is described in the literature by Tokumasu and Kato "Variation of Chromosome Numbers and Essential Aid Components of Plants Derived from Anther Culture of the Diploid and the Tetraploid in Pelargonium Rozeum", Euphytica, 28; 329, 1978, and by Pillai and Hildebrandt, "Induced Differentiation of Geranium Plants from Undifferentiated Callus In Vitro", Amer. J. Bot., 56: 52, 1969. The problem with the use of the filter paper bridge and test tube method is that the bridge portion of the paper holding the callus does not have a firm support and therefore the filter paper bridges have to be carefully folded so that the legs of the filter paper wicks are positioned along the walls of relatively narrow culture vessels, like test tubes, to provide a support for the tissue culture. This design makes it impractical to culture a large number of samples under identical conditions in one vessel, since a different vessel must be provided for each of the cultures. A problem also occurs in that only a certain filter paper materials are suitable for use in this method, since when the paper bridge is wet, the combined weight of the absorbed liquid nutrient medium and the cultured plant tissue specimen can collapse the bridge. Even for commonly available types of filter paper, the use of the filter paper bridge method is limited to cultures of small size since the growing culture can often by itself collapse the bridge thereby dumping the culture into the nutrient medium. It is also common, because there is not regulatory mechanism for the removal of excess liquid, for the tissues cultivated under such a method to become overly soaked with liquid medium to thereby suffer from inadequate gas exchange with the atmosphere to the detriment of the culture. Thus while this method is useful in a laboratory on small scale experiments, the adaptation of such a system to the larger scale cultivation of plant tissue cultures is not practical.
It has also been known previously in the technology that whole plants, as opposed to plant tissue cultures or callus cultures, can be cultivated in apparatus wherein the liquid nutrient requirements of the plant are met by use of a wick providing capillary action from a nutrient medium located beneath the plant. For example, the disclosure of U.S. Pat. No. 4,299,054, to Ware, describes a hydroponic assembly for growing plants which includes a wafer upon which the plant is grown. One or more wicks are provided depending from the wafer into a nutrient media provided in the apparatus so that liquid nutrient media travels by capillary action up the wick to saturate the wafer to provide liquid requirements for the plants needs. Such a system, while possibly satisfactory for the growth of an intact plant including roots, is not satisfactory for the growth of a plant explant tissue cultures or callus cultures since such a system requires root growth into the liquid culture medium for continued plant viability.